Cycles: Emission Shader does not bring enough light
The reason your scene is so dark and don't see any light, is because you are clamping all of the Direct and Indirect light to 0.5:



What that means is that NO LIGHT WILL BE BRIGHTER THAN 0.5 (0 being black, 1 being White).
Even if you make the lights extremely bright they will never be white!

You where probably trying to cut out fireflies, but by bringing the values so low you are clipping all of your highlights too!



With more reasonable Clamp Values you can regain control of your lights. Now the emitters on your lamps can be brought down: